As well known, in a grinding machine, the wheel is cyclically dressed, with simultaneous restoring of its shape by the so called diamond truing; the position of cutting surface of inoperative grinding wheel, relative to machine is determined by the position of diamond and by the traverses of wheelhead unit.
The surface which is being ground is identified by its rotation axis and by the diameter, and in the course of grinding operation, it is tangent to wheel cutting surface; however, the position of operating wheel differs from that which would be taken by the inoperative wheel, due to the presence of substantial elastic deformations, caused by the pressure of wheel against the workpiece, and that are usually, though improperly called "extension flexure" (which is actually a part only of such deformations).
With the method based on the grinding wheel control ("size-matic"), the gauging of hole depends on the reproducibility of hereinafter stated factors: (a) The position of truing diamond relative to workpiece; (b) The traverses of wheelhead unit; (c) The final extension flexure; (d) The wear of grinding wheel from the last diamond truing and the end of grinding operation. A periodic control of workpieces, with possible corrections, is required by the point (a), due to the action of slow drifts, caused by the wear and by thermic motions while, when the workpiece is fitted on slippers, the method becomes quite inapplicable, since any difference in the outer diameter should directly be reflected on the diameter of hole. The point (b) may be considered as complied with in the properly designed machines. By the point (c), final flexures so small are required, that the variations thereof may be neglected, and thus very long times for the extinction of ray of sparks. By the point (d), a diamond truing is necessary before of finishing pass.
With the methods based on the workpiece control ("gage-matic"), the direct action of stated factors on the gauging of hole is prevented, without however preventing an indirect action thereof. As a matter of fact, the final extension flexure cannot be controlled by such methods, and since such flexure results in a misalignment of grinding wheel, with a consequent taper of hole (above all in short, shape-ground holes, or ground with a small oscillation only), it follows that the gauging of hole is indirectly influenced by the final extension flexure, through alterations of the shape. Moreover, all variations in the final extension flexure result in differences in the finish degree of ground surfaces.
Additionally, no indication that may be utilized for a better exploitation of machining time can be obtained from either methods as stated above.